High Degree Of Polarization Research Articles

Spin polarization in the vicinity of quantum point contact with spin-orbit interaction

We have developed a novel technique for detection of spin polarization with a quantum dot weakly coupled to the objective device. The disturbance to the object in this technique is very small since the detection is performed through sampling of single electrons in the object with very slow rate. We have applied the method to a quantum point contact (QPC) under a spin-orbit interaction. A high degree of spin polarization in the vicinity of the QPC was detected when the conductance stayed on a plateau at a half of the unit conductance quantum ($G_{\rm q}/2\equiv e^2/h$), and also on another plateau at $2e^2/h$. On the half-quantum plateau, the degree of polarization $P$ decreased with the bias source-drain voltage of the QPC while $P$ increased on the single-quantum plateau, manifesting that different mechanisms of polarization were working on these plateaus. Very long spin relaxation times in the detector quantum dot probably due to dynamical nuclear spin polarization were observed. Anomalous decrease of $P$ around zero-bias was observed at a Kondo-like resonance peak.

Relationship between polarization characteristics and hemolysis rate and its potential application

Mueller polarimetry has been widely investigated in biomedical field. However the application in hemolysis monitoring is unavailable. This study deals with the backscattering polarimetric characterization of erythrocyte suspensions in low-osmolarity-induced hemolysis condition. Scattering and absorption are observed to decrease with increasing hemolysis. Increasing degree of polarization (DOP) corresponds to a decrease of scattering; increasing diattenuation corresponds to a decrease of scattering. However the decreasing absorption increases first and then decreases DOP in erythrocyte suspensions which are typical of Mie scatterers; reducing diattenuation corresponds to a decrease of absorption. Hence it is demonstrated that higher DOP is preserved in more serious hemolysis condition. In addition, DOP increasing trends are different in the conditions of below/above 6% hemolysis rate. Diatteunation shows an increasing trend at increase of blood hemolysis percentage with an exception when the hemolysis rate ranges from 5% to 6%. These results may be helpful for monitoring of hemolysis by polarimetric optical method.

NEAR-INFRARED CIRCULAR AND LINEAR POLARIMETRY OF MONOCEROS R2

ABSTRACT We have conducted simultaneous JHK s -band imaging circular and linear polarimetry of the Monoceros R2 (Mon R2) cluster. We present results from deep and wide near-infrared linear polarimetry of the Mon R2 region. Prominent and extended polarized nebulosities over the Mon R2 field are revisited, and an infrared reflection nebula associated with the Mon R2 cluster and two local reflection nebulae, vdB 67 and vdB 69, is detected. We also present results from deep imaging circular polarimetry in the same region. For the first time, the observations show relatively high degrees of circular polarization (CP) in Mon R2, with as much as approximately 10% in the K s band. The maximum CP extent of a ring-like nebula around the Mon R2 cluster is approximately 0.60 pc, while that of a western nebula, around vdB 67, is approximately 0.24 pc. The extended size of the CP is larger than those seen in the Orion region around IRc2, while the maximum degree of CP of ∼10% is smaller than those of ∼17% seen in the Orion region. Nonetheless, both the CP size and degree of this region are among the largest in our infrared CP survey of star-forming regions. We have also investigated the time variability of the degree of the polarization of several infrared sources and found possible variations in three sources.

Light Emission Characteristics of Nonpolar <inline-formula> <tex-math notation="LaTeX">$a$ </tex-math> </inline-formula>-Plane GaN-Based Photonic Crystal Defect Cavities

In this paper, the light emission performance from nonpolar $a$ -plane gallium nitride (GaN)-based photonic crystal (PC) slabs with line-type defect cavities has been investigated. Via the focused ion beam milling technique, thin membrane structures of the nonpolar GaN PC defect cavities were engineered, exhibiting one dominated resonant mode occurred at 419.3 nm with a high quality factor of $1.9\times 10^{3}$ in the photoluminescence emission characterizations. In the far-field region, light emission performance shows a high polarization degree of 50% with the beam direction perpendicular to the defect cavity. Compared with the PC defect cavity patterns fabricated on $c$ -axial oriented GaN materials, the $a$ -plane PC defect cavities display not only the linear light output–input power dependence, but also an invariant resonant mode of constant light emission behavior with increasing the injection carriers, therefore providing a new perspective for the future development of promising optoelectronic devices.

A wind-driving disc model for the mm-wavelength polarization structure of HL Tau

The recent advent of spatially resolved mm- and cm-wavelength polarimetry in protostellar accretion discs could help clarify the role of magnetic fields in the angular momentum transport in these systems. The best case to date is that of HL~Tau, where the inability to produce a good fit to the 1.25-mm data with a combination of vertical and azimuthal magnetic field components was interpreted as implying that centrifugally driven winds (CDWs) are probably not a significant transport mechanism on the $\sim 10^2\,$au scale probed by the observations. Using synthetic polarization maps of heuristic single-field-component discs and of a post-processed simulation of a wind-driving disc, we demonstrate that a much better fit to the data can be obtained if the radial field component, a hallmark of the CDW mechanism, dominates in the polarized emission region. A similar inference was previously made in modelling the far-infrared polarization map of the pc-scale dust ring in the Galactic centre. To reconcile this interpretation with theoretical models of protostellar discs, which indicate that the wind is launched from a comparatively high elevation above the mid-plane, we propose that most of the polarized emission originates -- with a high ($\ga 10$\%) intrinsic degree of polarization -- in small ($\la 0.1\,$mm) grains that remain suspended above the mid-plane, and that the bulk of the mm-wavelength emission is produced -- with low intrinsic polarization -- by larger grains that have settled to the mid-plane.

Predicting polarization signatures for double-detonation and delayed-detonation models of Type Ia supernovae

Calculations of synthetic spectropolarimetry are one means to test multi-dimensional explosion models for Type Ia supernovae. In a recent paper, we demonstrated that the violent merger of a 1.1 and 0.9 M$_{\odot}$ white dwarf binary system is too asymmetric to explain the low polarization levels commonly observed in normal Type Ia supernovae. Here, we present polarization simulations for two alternative scenarios: the sub-Chandrasekhar mass double-detonation and the Chandrasekhar mass delayed-detonation model. Specifically, we study a two-dimensional double-detonation model and a three-dimensional delayed-detonation model, and calculate polarization spectra for multiple observer orientations in both cases. We find modest polarization levels ($<$ 1 per cent) for both explosion models. Polarization in the continuum peaks at $\sim$ 0.1$-$0.3 per cent and decreases after maximum light, in excellent agreement with spectropolarimetric data of normal Type Ia supernovae. Higher degrees of polarization are found across individual spectral lines. In particular, the synthetic Si ii {\lambda}6355 profiles are polarized at levels that match remarkably well the values observed in normal Type Ia supernovae, while the low degrees of polarization predicted across the O i {\lambda}7774 region are consistent with the non-detection of this feature in current data. We conclude that our models can reproduce many of the characteristics of both flux and polarization spectra for well-studied Type Ia supernovae, such as SN 2001el and SN 2012fr. However, the two models considered here cannot account for the unusually high level of polarization observed in extreme cases such as SN 2004dt.

Low threshold continuous-wave lasing of yellow-green InGaN-QD vertical-cavity surface-emitting lasers.

Low threshold continuous-wave (CW) lasing of current injected InGaN quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs) was achieved at room temperature. The VCSEL was fabricated by metal bonding technique on a copper substrate to improve the heat dissipation ability of the device. For the first time, lasing was obtained at yellow-green wavelength of 560.4 nm with a low threshold of 0.61 mA, corresponding to a current density of 0.78 kA/cm2. A high degree of polarization of 94% were measured. Despite the operation in the range of "green gap" of GaN-based devices, single longitudinal mode laser emission was clearly achieved due to the high quality of active region based on InGaN QDs and the excellent thermal design of the VCSELs.

Polarization properties of OH emission in planetary nebulae

We present interferometric, full-polarization observations of the four ground-state transitions of OH, toward five confirmed and one candidate OH-emitting planetary nebulae (OHPNe). OHPNe are believed to be very young PNe, and information on their magnetic fields (provided by their polarization) could be key to understand the early evolution of PNe. We detect significant circular and linear polarization in four and two objects, respectively. Possible Zeeman pairs are seen in JaSt 23 and IRAS 17393-2727, resulting in estimates of magnetic field strengths between 0.8 and 24 mG. We also report the new detection of OH emission at 1720 MHz toward Vy 2-2, making it the third known PN with this type of emission. We suggest that younger PNe have spectra dominated by narrow maser features and higher degrees of polarization. Shock-excited emission at 1720 MHz seems to be more common in PNe than in early evolutionary phases, and could be related to equatorial ejections during the early PN phase.

\(e^{+} e^{-} \rightarrow \mathit{hZ}\) Collision in Randall-Sundrum Model

We study the production of Higgs and Z bosons which has been proposed as an option of \(e^{+}e^{-}\) collision at the ILC, CLIC with the polarization of the electron and positron beams in Randall-Sundrum model (RSM). The differential cross-sections are presented and numerical evaluation is given. Based on the results, we show that the advantageous direction to collect Higgs is perpendicular to the direction of the initial \(e^{-}\) beam. With the high integrated luminosity and at the high degree of polarization, the reaction can give observable cross-sections in future accelerators (ILC, CLIC).

Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology.

Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor-polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

Helicity-selective phase-matching and quasi-phase matching of circularly polarized high-order harmonics: towards chiral attosecond pulses

Phase matching of circularly polarized high-order harmonics driven by counter-rotating bi-chromatic lasers was recently predicted theoretically and demonstrated experimentally. In that work, phase matching was analyzed by assuming that the total energy, spin angular momentum and linear momentum of the photons participating in the process are conserved. Here we propose a new perspective on phase matching of circularly polarized high harmonics. We derive an extended phase matching condition by requiring a new propagation matching condition between the classical vectorial bi-chromatic laser pump and harmonics fields. This allows us to include the influence of the laser pulse envelopes on phase matching. We find that the helicity dependent phase matching facilitates generation of high harmonics beams with a high degree of chirality. Indeed, we present an experimentally measured chiral spectrum that can support a train of attosecond pulses with a high degree of circular polarization. Moreover, while the degree of circularity of the most intense pulse approaches unity, all other pulses exhibit reduced circularity. This feature suggests the possibility of using a train of attosecond pulses as an isolated attosecond probe for chiral-sensitive experiments.

41-2:Invited Paper: Quantum Dots and Aligned Quantum Rods for Polarized LC Backlights

A backlight that emits linearly polarized light would make it possible to increase the transmission through the first polarizer by a factor two. We used quantum rods that have a CdSe core that emits partially polarized yellowish green light (around 570 nm) and a CdS bulk that absorbs blue. The quantum rods can be aligned with their long axis parallel to an applied electric field with sufficient amplitude. The resulting layer emits linearly polarized light with a high degree of polarization. Such a layer can be used in different backlight configurations.

Investigation of the d(γ,n)p reaction for gamma beam monitoring at ELI-NP

The Extreme Light Infrastructure - Nuclear Physics facility will deliver brilliant gamma beams with high spectral density and a high degree of polarization starting in 2018 in Bucharest-Magurele, Romania. Several monitoring instruments are proposed for measuring the spectral, temporal, and spatial characteristics of the gamma beam. The d(γ,n)p reaction has been investigated for its use in determining the gamma beam parameters in a series of measurements carried out at the High Intensity Gamma Source, Durham, U.S.A.. Measurements of the emitted neutrons have been performed using liquid scintillator and 6Li-glass neutron detectors at several incident gamma energies between 2.5 to 20 MeV . The experimental results presented in this paper have shown that an instrument based on the d(γ,n)p reaction can be used to monitor the intensity and polarization of the gamma beam to be produced at ELI-NP.

Spectrum splitting for fast polarization switching ofundulator radiation.

A simple scheme to quickly switch the polarity of circular radiation is proposed, which is based on spectrum splitting of undulator radiation. In this scheme, two helical undulators with opposite helicities are placed tandem in one straight section, both of which are divided into several segments. The optical phases between segments are tuned so that light waves from one of the two undulators are out of phase, while those from the other are in phase. Then the radiation spectrum of the former is split and the intensity at the fundamental photon energy vanishes. As a consequence, the monochromated photon beam at the fundamental energy is circularly polarized with the helicity specified by the in-phase undulator, which can be quickly flipped by tuning the optical phase. Numerical calculations carried out to demonstrate the feasibility of the proposed scheme show that a relatively high degree of circular polarization is expected if the angular acceptance of the beamline is not too large.

Investigation of spin-gapless semiconductivity and half-metallicity in Ti2MnAl-based compounds

The increasing interest in spin-based electronics has led to a vigorous search for new materials that can provide a high degree of spin polarization in electron transport. An ideal candidate would act as an insulator for one spin channel and a conductor or semiconductor for the opposite spin channel, corresponding to the respective cases of half-metallicity and spin-gapless semiconductivity. Our first-principle electronic-structure calculations indicate that the metallic Heusler compound Ti2MnAl becomes half-metallic and spin-gapless semiconducting if half of the Al atoms are replaced by Sn and In, respectively. These electronic structures are associated with structural transitions from the regular cubic Heusler structure to the inverted cubic Heusler structure.

Spin-Polarized Light Emitting Self-Assembled InAs/GaAs Quantum-Dot Molecular Structures: The Dominant Mechanism for Spin Loss during Spin Injection

While conventional electronics only relies on manipulation of electronic charge, a promising direction for next-generation electronic and photonic devices is expected to benefit additionally from exploiting the spin degree of freedom leading to e.g. spin-FET, spin lasers and spin-LEDs. Among vital ingredients in achieving spin functionalities are efficient injection of spin polarized carriers or excitons, prolonged spin dephasing/relaxation time as well as our ability to manipulate spin in a desirable way. Semiconductor quantum dots (QDs) have emerged as an excellent materials system in this context, as motion-induced spin relaxation that is dominant in bulk and 2D semiconductor structures is quenched due to the three-dimensional confinement. Combining with their high radiative recombination efficiency, QDs are good candidates for spin light-emitting devices and strongly polarized light sources. Indeed, appreciably long spin lifetimes have been reported in semiconductor QDs and a high degree of electron spin and optical polarization has also been demonstrated through optical orientation under resonant excitation of QD excitons. However, upon spin injection from adjacent layers such as a wetting layer (WL) and a barrier layer, that is a necessary step required for operation of most envisaged spintronic devices based on QDs, reported values of electron spin and optical polarization degree of QDs remain very low so far. The exact physical mechanism for the observed low spin injection efficiency is still not understood, unfortunately, though it was generally believed to be associated with accelerated spin relaxation in WL and barrier layers when carrier and exciton motions (i.e. with a non-zero momentum k ) promote spin-orbit mediated spin relaxation. Up to now, WL and barrier layers surrounding QDs are commonly regarded as being of “ideal” 2D and 3D characters, respectively. In this work we examine the exact path of spin injection from WL and barriers into QDs in self-assembled InAs/GaAs QDs and lateral QD molecular structures (QMSs), by employing optical orientation, single-dot photoluminescence (PL) and PL excitation spectroscopy. The studied QDs and QMSs include single quantum dots (SQDs), laterally-aligned double QDs (DQDs), quantum rings (QRs), and quantum clusters (QCs). We find, surprisingly, that exciton spin injection in these QDs and QMSs is dominated by localized excitons confined within the QD-like regions of the WL and GaAs barrier layer immediately surrounding QDs and QMSs that in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We identify the microscopic origin of the observed severe spin loss during spin injection as being due to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We find that the AEI of the injected excitons and thus the efficiency of the spin injection processes are directly correlated with the overall geometric symmetry of the QMSs, namely, spin injection is more efficient in the QMSs with a higher symmetry in geometric arrangement such as QRs and QCs as compared with the lower-symmetry DQDs. This correlation is attributed to the fact it is the symmetry of the QMSs that largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier, which governs the AEI. This work represents a significant advance in our understanding of the spin injection processes and spin loss mechanism in QDs and QMSs, and it also provides a useful guideline in designing strategies to improve spin injection efficiency by optimizing the lateral arrangement of the QMSs, thereby overcoming a major obstacle in utilizing semiconductor QDs and QMSs for device applications in spintronics and spin photonics.

Structural design and optimization of near-ultraviolet light-emitting diodes with wide wells

The characteristics of the near-ultraviolet (NUV) light-emitting diodes (LEDs) with wide (14-nm-thick) and narrow (2-nm-thick) wells under the situations of different numbers of wells and degree of polarization are systematically investigated. The simulation results show that the Auger recombination can be efficiently suppressed with the increase of number of wells in NUV LEDs. For the LEDs with wide wells, the quantum-confined Stark effect and Shockley–Read–Hall recombination play an important role when the number of wells increases, especially when the LED is under low current injection or high degree of polarization. In order to take the advantage of using wide wells, it is proposed that the quaternary Al0.1In0.05Ga0.85N barriers be used in wide-well NUV LEDs along with the use of Al0.3Ga0.7N/Al0.1Ga0.9N superlattice electron-blocking layer to mitigate the polarization effect and electron overflow. With this band-engineering structural design, the optical performance of the wide-well NUV LEDs is much better than its thin-well counterpart even under the situation of high degree of polarization.

Acetylene Oligomerization over Pd Nanoparticles with Controlled Shape: A Parahydrogen-Induced Polarization Study

Substantial signal enhancements achieved by using parahydrogen in catalytic hydrogenations are powerful tools for mechanistic studies of chemical reactions involving molecular H2. Potentially, this technique can be extended to other reaction classes, providing new information about reaction mechanisms. Moreover, this can lead to new substances with highly polarized spins. Herein, we report strong signal enhancements of oligomerization reaction products observed during the selective hydrogenation of acetylene over Pd nanoparticles of different shapes and sizes supported on SiO2. C4 oligomeric products (1,3-butadiene, 1-butene, 2-butene) demonstrated a high degree of nuclear spin polarization, with the highest degree observed for 1-butene [more than 1.7% vs (2.4 × 10–3)% at thermal equilibrium], which was an order of magnitude larger than that of the triple C—C bond hydrogenation products. No dependence of polarization on the metal surface statistics or, generally, on the nanoparticle morphology (cubic, oct...

Modeling the polarization of high-energy radiation from accreting black holes

The high-energy emission ($400$ keV $- 2$ MeV) of Cygnus X-1 --the most well-studied Galactic black hole-- was recently found to be strongly polarized. The origin of this radiation is still unknown. In this work, we suggest that it is the result of non-thermal processes in the hot corona around the accreting compact object, and study the polarization of high-energy radiation expected for black hole binaries. Two contributions to the total magnetic field are taken into account in our study, a small scale random component related to the corona, and an ordered magnetic field associated with the accretion disk. The degree of polarization of gamma-ray emission for this particular geometry is estimated, as well as the angle of the polarization vector. We obtain that the corona+disk configuration studied in this work can account for the high degree of polarization of gamma-rays detected in galactic black holes without the need of a relativistic jet; specific predictions are made for sources in a low-hard state. In particular, the model is applied to the transient source \xtee; we show that if a new outburst of \xte is observed, then its gamma-ray polarization should be measurable by future instruments, such as ASTRO-H or the proposed ASTROGAM.

A Quantitative Coarse-Grained Model of Sugars

Sugars, in particular glucose, are a major component of biomass and are present in a wide range of biomolecules, but have been relatively neglected in force field development. In particular there are no existing coarse-grained models capable of describing the conformational changes or high degree of polarity which are important in determining their structure and behaviour. Traditional atomistic models fare a little better, but still do not adequately reproduce many properties. Furthermore, the majority of sugar models are parametrized only for glucose, prohibiting their application to more complex molecules such as lipopolysaccharide.In this work we have developed a high quality coarse-grained description of each stereoisomer of the aldohexoses, incorporating conformational flexibility and a sound description of electrostatics through monopoles and rotating dipoles. We show significant speedup of simulations with respect to fully atomistic models while maintaining, and in some cases improving on, the quality of description. Additionally, ELBA is compatible with the AMBER fully atomistic force field, allowing dual-resolution simulations for a balance between atomistic detail and coarse-grained efficiency.Finally, a set of tools has also been developed to aid in the parametrization and analysis of coarse-grained molecular dynamics simulations.